Download Creating floodplain wetland features

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CREATING FLOODPLAIN
WETLAND FEATURES
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7.2 Floodplain wetland mosaic
RIVER THAMES
LOCATION - Pinkhill Meadow, Farmoor Reservoir, Oxfordshire SP 439067
DATE OF CONSTRUCTION - JUNE 1990, December 1991
Environment Agency
AREA – 1.5ha
instrumentation building
for gauging station
COST - £112,000
N
Experimental
ponds
Towpath
Grazing
Meadow
es
Reed bed
er
Riv
am
Th
Wet
Meadow
Gravel area
Wader
scrape
Mudbank
Main pond and
Gravel island
Reed pool
Reed
pool
Public
footpath
Reed
bed
Thorney scrub
Farm
oor
PLAN
Birdwatching
hide
Rese
rvoir
Figure 7.2.1
PINKHILL MEADOW
OF
DESCRIPTION
Pinkhill Meadow is located in a 4 hectare meander of
the River Thames at Farmoor Reservoir. In a detailed
landscape assessment of the reservoir site in 1988
the meadow was identified for its potential for
wetland creation. Few areas of wetland had survived
agricultural improvement in this part of the Upper
Thames Valley. Approx. 1.5 hectares of the meadow
still had a valuable relic meadow flora including
species such as Adders Tongue, Great Burnett and
Pepper-saxifrage.
The aim of the scheme was to compliment the river
and the reservoir habitats by restoring the floodplain
wetland within the meadow which had been largely
disturbed during reservoir construction.
A key objective was to restore habitat for breeding
7.2
(page 1 of 4)
Re
perim servoir
eter
road
waders and wildfowl, notably Redshank, and to
create a place where people could experience a wide
variety of wetland wildlife at close quarters and enjoy
a more “natural” floodplain landscape.
The project was a collaboration between the
landowners (Thames Water) and the National Rivers
Authority, advised by Pond Action. Large parts of the
Farmoor wetland complex are open to members of
public, and keys to the birdwatching hides can be
obtained from the Wardens Lodge at Gate 3.
DESIGN
A concept plan for the site was prepared from the
landscape appraisal and developed into the detailed
design, incorporating the project objectives with the
site constraints.
These techniques were developed to suit site specific criteria and may not apply to other locations
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CREATING FLOODPLAIN
WETLAND FEATURES
Marginal Reed Bed
Undulating topography
+0.2 – -0.2m below GWL
Reed Pool
-1.0m below GWL.
Barrier to reedbed
extension
Existing
1.0m
ground level
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Wader Scrape
Very gentle undulating topography
above groundwater. Surface water fed feature.
max depth of retained water 0.4m
Temporary
water level
GWL
Slope
varies 1:1 to 1:3
1.0m max depth
Figure 7.2.2
INDICATIVE SECTION THROUGH
REED POOL
The mosaic of over 40 ponds and pools was designed
to maximise the topographical, hydrological, biological
and visual diversity of the site. This included specific
creation of individual waterbodies with a wide range
of maximum depths and permeability, and low angle,
undulating drawdown zone areas to encourage both
wetland plant diversity and wading bird habitat.
Two phases of excavation were undertaken, the first
in June and July 1990 and the second over the winter
period in 1991/2. By phasing the works it was possible
to better understand the detail needed for the more
complex works in the second phase.
Excavation was based on detailed landscape design
drawings provided by the NRA landscape architect,
and firmly led by close project management and
continuous on-site supervision from key members
of the project team. In this way the inexperienced
machine operator was able to achieve the very
subtle variations in topography in relation to water
levels. The 20,000m3 of excavated material was
carefully graded into a low hill near the adjacent
Pinkhill Lock, but outside of the floodplain. This
was then planted with trees and shrubs and sown
with a wildflower seed mix.
In phase 1 four waterbodies were created; the main
pond, wader scrape and two reedbed pools. In phase 2
the existing waterbodies were extended, added to
and re-profiled to create areas of shallow water, wet
meadow, mudflat and temporary pool habitat.
Observations of the phase 1 works provided valuable
detail for the improvements undertaken in phase 2.
Observations of actual as opposed to design waterlevels in the pools were used to refine the new
excavation levels, marginal areas and undulating
contours of the wet meadow. The location of the
mudflats was also based on the usage of the various
areas of the site by different target wader species.
Key features created:
Deep water
The main pond is up to 2.5m deep and covers an
area of just under 0.5ha and was excavated down
into the gravel aquifer. The size and depth increases
the diversity of habitats and isolates the several
islands reducing the likelihood of predation of bird
nests. The depth also ensures open water and from
a management viewpoint it also restricts the
complete colonisation by marginal wetland plants.
Shallow-water areas and edges
These areas were designed to between 300mm
below and 100mm above normal water levels.
As the main pond level will fluctuate by about
300mm, reflecting groundwater levels, these areas
are important to retain shallow slopes at the
water’s edge.
Wader scrapes
(February 1992)
These techniques were developed to suit site specific criteria and may not apply to other locations
(page 2 of 4)
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Main pond, pool and
reedbed creation
(February 1992)
Temporary ponds and pools
The site also includes small temporary pools, some
isolated and some bordering the larger waterbodies.
Sizes vary from a few square meters to the larger
two semi-permanent ponds (approx. 100m2). These
transient ponds are designed to dry out in drought
years (two or three times since excavation) and
provide a habitat with low fish predation, benefiting
many aquatic invertebrates and some amphibians.
these species. Selectively placed cobbles and
boulders also provide surface variation.
Wader scrape
A 400mm maximum depth shallow pool was formed
within the alluvium overlaying the gravel aquifer and
the water level controlled by means of a connecting
pipe to the main pond. This feature provides
extensive muddy margins for feeding waders and
Teal, particularly during autumn migration.
Undulating wet grassland
Small variations in topography were engineered
to create an undulating meadow with water levels
close to the surface (between 100 and 200mm above
normal water level). This marshy/tussocky Rush
and Sedge dominated area was designed to provide
feeding and nesting areas for waders, particularly
Redshank and Snipe.
Mudflats and islands
These were created by excavating into the alluvium.
Gentle slopes of 1:20 min. provide bare feeding
and nesting habitats for wading wildfowl, but also
created a more open habitat suitable for some
marginal wetland plants.
Gravel islands and margins
Created over an area of 0.1ha, these provide nesting
habitat for Little Ringed Plover and Common Tern.
The gravel was carefully selected from a local source
to ensure a good size distribution, important for
Reedbeds
Two linear reedbeds, totalling over 250m in length,
were excavated along the eastern edge of the site.
Shallow trenches were dug and planted with pot
Figure 7.2.3
INDICATIVE
SECTION THROUGH MAIN POND
AND GRAVEL ISLANDS
Simple pipe
connection to
scrape to
top up levels
Very gentle draw down
zone creating muddy edges
to pond used by waders
(in places colonised
by marginal plants)
Main Pond
Deep open water
1.0m – 2.5m below GWL
Gravel Islands
Max 0.5m above GWL.
Replenised with local gravel every 2–3 years
Existing ground level
GWL
7.2
(page 3 of 4)
These techniques were developed to suit site specific criteria and may not apply to other locations
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CREATING FLOODPLAIN
WETLAND FEATURES
Figure 7.2.4
INDICATIVE
T
7
SECTION THROUGH WET GRASSLAND
Mudbanks
Main
Pond -0.2m – +0.2m
above and below GWL
Shallow water
channel
-0.5m below GWL
Existing
ground level
Wet Grassland
Very gentle undulating topography
-0.3m – 0.8m below existing ground level
GWL
Temporary pools
grown Common Reed. These serve as a boundary
to human disturbance from adjacent footpaths and
provide valuable nesting and foraging habitat for
wetland birds such as Reed Warbler and Water Rail.
Scrub
A double row of mixed shrubby Willow (incorporating
some Hawthorn, Blackthorn and Dog Rose) forms
a 4m wide hedge at the eastern edge of the site
linking adjacent areas of reed, meadow, hedges and
woodland. As with the reedbed the presence
of the hedge also helps to mitigate the disturbance
of the nearby footpath, as well as sustaining a rich
insect population and providing over-wintering
cover for frogs, toads and newts. The scrubby
character is to be retained by staggered coppicing
on an annual basis ensuring a permanent screen
is maintained.
SUBSEQUENT PERFORMANCE 1999 – SPRING 2001
Continuous post-project appraisal was carried out
on this site for the first 5 years after construction and
the results showed that this small wetland creation
scheme quickly acquired an extremely rich wildlife
community.
In these 5 years over 20% (over 60 species) of all
Britain’s wetland and aquatic plant species colonised
the site. In the main pond alone the plant community
was one of the richest recorded in ponds in the county.
Similarly 22% (over 150 species) of Britain’s macroinvertebrate species were recorded on the site, including
12 breeding species of dragonfly.
Breeding wader densities have been very high, in
one year up to 100pairs/km2 equalling that of grazing
marsh and other important British wader habitats.
In 1993 and 1994 two pairs of Little Ringed Plover
bred, representing 15% of Oxfordshire’s breeding
population . Unfortunately the site was too small
to sustain such densities and the plovers have not
returned since 2000.
As a result of the commitment of the partners
and individuals involved, the continued appraisal
of the site’s development some minor and major
modifications have been funded in every second or
third year between 1992 and 2002.
These have included:
• managing gravel islands;
• scraping new mudflats;
• creating new pools;
• doubling the size of the main reedbed;
• annual coppicing and thinning of the willow scrub.
A key reason for the success of Pinkhill is the
combination of critical factors for creating
biologically high quality sites:
• good water quality;
• high degree of landscape connectivity to other
wetlands;
• complex mosaic design integrated sensitively into
the floodplain;
and specifically at Pinkhill;
• wetland creation with people in mind;
• combining 6 Key River Thames habitats in
one place;
Similar design principles as those used to create
Pinkhill Meadow have subsequently been adapted to
create two further wetlands at the site, known as
Shrike Meadow and Buckthorn Meadow.
Contacts:
Richard Hellier, Environment Agency – Thames Region,
Kings Meadow House, Kings Meadow Road,
Reading, Berks, RG1 8DQ.
Tel: 01189 535000.
Conservation and Heritage Team,
Thames Water, Clearwater Court (RBH2),
Western Road, Reading, Berks, RG1 8DB
Tel: 01189 593720.
Ponds Conservation Trust,
c/o Oxford University,
Gipsy Lane, Headington,
Oxford, OX3 0PB.
Tel: 01865 483278.
Aerial view of site
These techniques are developed to suit site specific criteria and may not apply to other locations
(page 4 of 4)
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